This invention pertains to the casting of metals in sand molds, and particularly to methods and materials for increasing the lives of the mold patterns which are employed therein.
The introduction of a molten metal into a cavity, or mold, where upon solidification, the resulting casting becomes an object whose shape was determined by the mold, is an old art. Equally as old is sand casting. In this molding process a wood, metal or plastic pattern is fabricated in the shape of the part to be produced. Sand is then compacted around the pattern in such a way that the top portion of the mold and the pattern can be removed, leaving a mold cavity the shape of the pattern. Molten metal is then poured into the mold cavity.
It is well known that to increase the life of a mold and to make the removal of the casting easier, the surfaces of the mold cavity must be coated with a protective material. In the case of sand castings however it is the pattern which must be coated. Prior art coating compositions however deal primarily with mold coatings rather than pattern coatings. Hence these materials will first be considered. A wide variety of mold coating compositions have been suggested, some as long ago as 1904. (See U.S. Pat. No. 772,440.) Most of these mold release agents contain a clay in one of its forms. For example, in 1925 a mold coating containing powdered fire clay, sodium silicate, and water was proposed in U.S. Pat. No. 1,561,561. Silica, alumina and vegetable oils are disclosed in U.S. Pat. No. 977,801. In U.S. Pat. No. 1,688,350 the method employed was to coat the mold surfaces with a mixture of fire clay and silica, and thereafter to treat the coating with crude oil.
As can be discerned from these early patents, clays were used either with water or with oils. Later developments involved the combination of clays with binders to yield mold coating compositions. Thus in Re 26,969 crystalline silica, alumina and the like were incorporated in binders such as colloidal silica sols, aluminum phosphate, or ethyl silicate. In U.S. Pat. No. 4,529,028 the binder was a resinous polymer. Bentonite and other clays were used as suspending agents for such refractory materials as fused silica or powdered zircon.
As pointed out in U.S. Pat. No. 4,443,259 coatings for foundry cores and molds are basically mold release agents. They are used to obtain smoother casting surfaces with fewer defects. In its simplest form, the patentee explains, such a coating is simply a suspension of bentonite, kaolin and other members of the montmorillonite group of clays in water.
As in the case of moldings, the use of sandcasting patterns is not without its own problems. The pattern surfaces erode and pit when successive mold cavities are produced using them. When such erosion occurs, molding sands have a greater tendency to adhere to the pattern when it is removed, affecting the mold cavity. Mineral seal oil and mineral seal oil-clay compositions appear to confer on sand casting patterns results not obtainable with other pattern coating materials. Accordingly in spite of available pattern coating compositions, mineral seal oil and mineral seal oil-clay coatings are still the commercial preference.
Even though mineral seal oil and mineral seal oil-clay mold coating compositions have been commercially successful, they are not without their disadvantages. Mineral seal oil is a petroleum distillate and a solvent. As such it has properties which are not entirely desirable, particularly a low flash point and some toxicity. Its flash point of 100.degree. to 300.degree. F. can lead to plant fires. Care must be excersized to avoid contact with strong oxidizing agents, open flames, and electric sparks. In addition its volatility is such that inhalation may cause local irritations, drowsiness, collapse, muscle twitching, coma, and, in some instances, pneumonia. By the practice of this invention the benefits of mineral seal oil-clay coating compositions are retained without its detriments.